RAID is an acronym for Redundant Array of Inexpensive (or
Independent) Disks. A RAID array is a collection of drives which
collectively act as a single storage system, which can tolerate
the failure of a drive without losing data, and which can operate
independently of each other.

What are the different RAID
levels?

A research group at UC-Berkeley coined the term
"RAID", defining six RAID levels. Each level is a
different way to spread data across multiple drives--a compromise
between cost and speed. Understanding these levels is important,
because each level is optimized for a different use.

RAID Level 0

RAID Level 0 is not redundant, hence does not truly fit the
"RAID" acronym. In level 0, data is split across
drives, resulting in higher data throughput. Since no
redundant information is stored, performance is very good, but
the failure of any disk in the array results in data loss.
This level is commonly referred to as striping.

RAID Level 1

RAID Level 1 provides redundancy by duplicating all data
from one drive on another drive. The performance of a level 1
array is only slightly better than a single drive, but if
either drive fails, no data is lost. This is a good
entry-level redundant system, since only two drives are
required; however, since one drive is used to store a
duplicate of the data, the cost per megabyte is high. This
level is commonly referred to as mirroring.

RAID Level 2

RAID Level 2, which uses Hamming error correction codes, is
intended for use with drives which do not have built-in error
detection. All SCSI drives support built-in error detection,
so this level is of little use when using SCSI drives.

RAID Level 3

RAID Level 3 stripes data at a byte level across several
drives, with parity stored on one drive. It is otherwise
similar to level 4. Byte-level striping requires hardware
support for efficient use.

RAID Level 4

RAID Level 4 stripes data at a block level across several
drives, with parity stored on one drive. The parity
information allows recovery from the failure of any single
drive. The performance of a level 4 array is very good for
reads (the same as level 0). Writes, however, require that
parity data be updated each time. This slows small random
writes, in particular, though large writes or sequential
writes are fairly fast. Because only one drive in the array
stores redundant data, the cost per megabyte of a level 4
array can be fairly low.

RAID Level 5

RAID Level 5 is similar to level 4, but distributes parity
among the drives. This can speed small writes in
multiprocessing systems, since the parity disk does not become
a bottleneck. Because parity data must be skipped on each
drive during reads, however, the performance for reads tends
to be considerably lower than a level 4 array. The cost per
megabyte is the same as for level 4.

The right choice depends on the application. The following
table summarizes the RAID levels with some of their possible uses.

RAID Level Uses

Level 0 (striping)

Any application which requires very high speed storage, but
does not need redundancy. Photoshop temporary files are a good
example.

Level 1 (mirroring)

Applications which require redundancy with fast random
writes; entry-level systems where only two drives are
available. Small file servers are an example.

Level 4 (parity)

Applications which require redundancy at low cost, or with
high-speed reads. This is good for archival storage. Larger
file servers are an example.

Level 5 (distributed parity)

Similar to level 4, but may provide higher performance if
most I/O is random and in small chunks. Database servers are
an example.

Often, it makes sense to use more than one level. For instance,
in a two-drive system, one partition could use level 0 to offer
the highest performance for temporary files; another partition
could use level 1 to offer security for important data or
applications. In a three-drive system, a partition for temporary
files could use level 0, the boot disk could use level 1, and
large data files could be stored on a level 4 partition.

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